Titan Mode

Titan Mode is a Dependent Perk offered if you have all three of Titan Heart, Titan Shield and Titan Armour (see this post for how they reduce damage taken). It’s almost always offered immediately after you get the 3rd pre-requisite, although sometimes you have to wait for the next roll (and very very rarely, the one after). In case you decline it (e.g. to pick something like multishot) you will usually get it again in the following (or subsequent) roll, so unless you are close to the end of the perk rolls for the run it’s not risky to defer it

When you have it, jumping on enemies deals damage of 100x Loadout Attack for regular enemies (including minions spawned by bosses) but only 5x for bosses.

When in Titan Mode (i.e. actually jumping on an enemy, not just having the perk) you are supposedly immune to damage from below but there have been cases (say, last boss in World 9) where the immunity has failed

It’s fun to do Titan mode on a boss just under a barrier - then your rebound is curtailed so you hit much more rapidly